This study proposes to study the immunogenicity of measles vaccine when it is given in two doses, one at 6 months of age and one at 12 months of age, and to compare the seroresponses of the two vaccinations to ascertain if a single vaccination given at 6 months is as immunogenic as two doses, or if a single vaccination given at 12 months is efficacious. This protocol is still active in terms of patient follow-up. Results of the study are being analyzed for statistical signifance.